Vehicle drive transmission and electrically assisted steering system

ABSTRACT

A transmission for a vehicle, particularly a skid-steered vehicle, that employs motive power from a prime mover delivered through an input shaft to drive left and right drive shafts at a nominal speed and input power from an electric motor to vary the speed of the left and right drive shafts according to steering commands from a steering control structure. The speed of the left and right drive shafts is directly related to a speed of the input shaft and the nominal speed of the left or right drive shaft is varied upwardly or downwardly by a ratio of the speed of the steering shaft via a speed varying structure. The speed of the left and right drive shafts is simultaneously varied in opposite directions (i.e. upwardly and downwardly) relative to the nominal speed by an equal number of rotations.

TECHNICAL FIELD

This disclosure relates to transmission and steering systems forvehicles, particularly skid-steered vehicles such as all-terrainvehicles (ATV's) propelled by wheels or an endless loop track on groundor water. More particularly, the invention relates to a vehicletransmission and steering system that is drivable by both a prime mover(e.g. combustion engine or electric motor) and an electric steeringmotor simultaneously, for example to facilitate skid-steering of thevehicle.

BACKGROUND

On skid-steered vehicles, such as ATV's (including amphibious ATV's), toaccomplish a turn to the left, for example, the drive structure (wheelsor tracks) on the left side is braked while power is transmitted to thewheels on the right. This leads to a waste of power through braking,slowing the vehicle through the turn and making the turn abrupt,imprecise and inefficient in terms of energy consumption.

Attempts have been made to provide improved transmissions forall-terrain skid-steered vehicles. For example, U.S. Pat. No. 8,439,152,filed Aug. 6, 2010, discloses a transmission with improved steeringsensitivity that makes use of a triple-differential arrangement.However, further improvements are required.

SUMMARY

In one aspect, there is provided a transmission for a vehicle comprisingan input shaft to be driven by a prime mover, a steer shaft to be drivenby a steering motor and speed varying structure to vary a rotationalspeed of a left and right drive shaft of the transmission according tothe rotational speed of the steering shaft.

The left and right drive shafts may each have a nominal speed that isdirectly related to a speed of the input shaft. The speed of the left orright drive shaft may be varied upwardly or downwardly relative to thenominal speed by a ratio of the speed of the steer shaft via the speedvarying structure. The ratio may be 0.5, 1, 1.5, 2, 2.5, 3, 3.5 or anyvalue in the range of 0.5 to 3.5. The speeds of the left and right driveshafts may be simultaneously varied in opposite directions (i.e.upwardly and downwardly) relative to the nominal speed by an equalamount.

In another aspect, there is provided a transmission for a vehiclesubstantially as shown and described herein.

In yet another aspect, there is provided a steering system for a vehiclecomprising the transmission substantially as shown and described herein,a steering motor and a steering control structure for providing steeringcommands to the steering motor.

In still another aspect, there is provided a vehicle comprising thetransmission substantially as shown and described herein.

Further features will be described or will become apparent in the courseof the following detailed description. It should be understood that eachfeature described herein may be utilized in any combination with any oneor more of the other described features, and that each feature does notnecessarily rely on the presence of another feature except where evidentto one of skill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

For clearer understanding, preferred embodiments will now be describedin detail by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 shows a perspective view of an embodiment of a transmission;

FIG. 2 shows a side view of the embodiment of the transmission of FIG.1;

FIG. 3 shows an end view of an embodiment of a transmission and steeringsystem, schematically illustrating a parking brake and steering motor;

FIG. 4 shows a first schematic diagram of the embodiment of thetransmission of FIG. 1 with a hi/lo selector in the NEUTRAL position;

FIG. 5 shows a second schematic diagram of the embodiment of thetransmission of FIG. 1 with the hi/lo selector in the LOW position;

FIG. 6 shows a third schematic diagram of the embodiment of thetransmission of FIG. 1 with the hi/lo selector in the HIGH position;

FIG. 7 shows a cross-sectional illustration of a lower portion of theembodiment of the transmission of FIGS. 1 and 3; and,

FIG. 8 shows a front portion of a vehicle including a transmissionaccording to FIGS. 1-7, which forms part of a steering system of thevehicle.

DETAILED DESCRIPTION

Like elements are described with like reference numerals; thus, itemsshown on a particular drawing may be described in connection withanother drawing containing the same reference numeral.

Referring generally to FIGS. 1-8 and starting specifically with FIG. 7,the transmission 10 shown in the drawings is contained in a transmissionhousing 20. In FIGS. 1-6, the transmission housing 20 is omitted tobetter show interior features of the transmission. Referring now to FIG.8, motive drive power is provided by a prime mover 130, such as acombustion engine or electric motor, and applied to an input shaft 23(see FIGS. 1-6). In some embodiments, the drive power from the primemover 130 is not applied directly to the input shaft 23, but rather istransmitted through a variable-speed drive 114, for example of themechanically geared, hydrostatic or continuously-variable transmission(CVT) type or, when the prime mover is an electric motor, of the voltagecontrolled, current controlled or variable-frequency (VFD) type.

Drive power passes through the transmission system from the input shaft23 to the left and right drive-shafts 25L,25R (FIGS. 1-7) via speedvarying structure in a manner to be described. The drive-shafts 25L,25Rmay be connected to the vehicle drive structure 105 (tracks in theembodiment of the vehicle shown in FIG. 8) directly or throughrespective final-ratio-drives (not shown).

Steering of the vehicle is effected by a steering system comprising thetransmission 10 and steering control structure 160 that provides signalsto a steering motor 165 that is connected to the transmission 10 in amanner to be further described. A steering command interface 162 (handlebars in the embodiment of the vehicle shown in FIG. 8) is used totransmit an operators steering commands to the steering controlstructure 160. It will be appreciated by those of skill in the art thatsteering command interface 162 could comprise a steering wheel or a pairof independent left/right skid-steer handles as is conventionally knownfor skid-steered vehicles. In addition, the vehicle need not beconfigured for transporting the operator, such as in an autonomous orremotely controlled vehicle. In this case, the steering commandinterface 162 may be remote from the vehicle and connected wirelessthereto. In the case of autonomous vehicles, the steering commandinterface 162 may be used to program the steering control structure 160via a wired or wireless interface and is not necessarily in continuouscommunication with the steering control structure 160.

Turning to FIGS. 1-7, the speed varying structure of the transmission 10will now be further described. An engine-high-gear 50 and anengine-low-gear 52 are carried on the input shaft 23 and are mounted ongear-bearings 51 for rotation relative to the input shaft 23. Alsocarried on the input shaft 23 is a hi/lo selector-sleeve 54, which issplined to the input shaft, i.e. the selector-sleeve 54 is constrainedto rotate with the input shaft 23, but can slide axially along the inputshaft. The input shaft 23 is rotationally supported within the housing20 by input-shaft-bearings 26L,26R

Changing between LOW and HIGH is accomplished by manually operating thehi/lo shift-rod 55. A hi/lo shift-fork 53, mounted on and fixed to thehi/lo shift-rod 55, engages the hi/lo selector-sleeve 54, and slides theselector-sleeve 54 along the splines of the input shaft 23. In turn,this movement dogs the selector-sleeve 54 either to the engine-high-gear50 or to the engine-low-gear 52. Thus, the driver is able to set thevehicle either in HIGH speed mode or in LOW speed mode through rotationof the hi/lo shift-rod 55. The hi/lo shift-rod 55 is supported at oneend within the housing 20 by shift-support-shaft 57 and an opposite endextends outwardly from the housing to facilitate easy rotation thereof.

When the hi/lo selector-sleeve 54 is moved to the left (as illustratedin FIG. 5) through counter-clockwise rotation of the hi/lo shift-rod 55,the selector sleeve 54 engages with the engine-low-gear 52, which isthereby forced to rotate with the input shaft 23. When the hi/loselector-sleeve 54 is moved to the right (as illustrated in FIG. 6)through clockwise rotation of the hi/lo shift-rod 55, theselector-sleeve 54 engages with the engine-high-gear 50, whereby now theengine-high-gear 50 is forced to rotate with the input shaft 23.

When the hi/lo selector-sleeve 54 is engaged with the engine-low-gear52, the selector-sleeve 54 is clear of the engine-high-gear 50, which istherefore free to rotate relative to the input shaft 23. That is to say:when LOW mode is selected, the engine-high-gear 50 is rotationally freeof the input shaft 23. Correspondingly, when HIGH mode is selected, i.e.when the hi/lo selector-sleeve 54 is engaged with (i.e. dogged to) theengine-high-gear 50, the selector-sleeve 54 is clear of theengine-low-gear 52, which is therefore free to rotate relative to theinput shaft 23.

An inter-shaft 38 is driven in a direction opposite to the rotation ofthe input shaft 23 at either a LOW or HIGH speed, depending on theposition of the hi/lo selector-sleeve 54. The range of the left-rightmovement of the selector-sleeve 54 includes a neutral portion (asillustrated in FIGS. 3 and 4) that is sufficiently large to prevent LOWand HIGH from being engaged at the same time.

The engine-high-gear 50 drives an intermediate-high-gear (orinter-high-gear) 58, which is solid with the intermediate-shaft (orinter-shaft) 38. The engine-low-gear 52 drives an intermediate-low-gear(or inter-low-gear) 63, which also is solid with the inter-shaft 38. Theinter-shaft 38 is rotationally supported within the housing 20 byinter-shaft-bearings 39L,39R.

The inter-high-gear 58 also doubles as a drive transmission gear, orinter-drive-gear, for conveying drive power to the drive-differentials42. The inter-high-gear/inter-drive-gear 58 meshes with awheel-drive-gear 45, which is solid with a wheel-ring 47 of thedrive-differentials 42. Thus, the wheel-drive-gear 45 and wheel-ring 47rotate in the same direction as the input shaft 23.

In a numerical example of the current embodiment, the engine-low-gear 52has twenty-five teeth, and the inter-low-gear 63 has forty-one teeth.Thus, when the input shaft 23 is driven at e.g. +1000 revolutions perminute (rpm), in LOW speed mode the inter-shaft 38 rotates at aninter-shaft speed of 1000*25/41=−610 rpm. (The negative sign indicatesrotation in the opposite sense to the rotation of the input shaft 23,which is described with a positive sign). The engine-high-gear 50 hastwenty-seven teeth, and the inter-high-gear 58 has twenty-seven teeth;thus, in HIGH speed mode when the input shaft 23 has an input shaftspeed of +1000 rpm, the inter-shaft speed is 1000*27/27=−1000 rpm.

Continuing with the numerical example, the wheel-drive-gear 45 hasseventy-three teeth and the inter-high-gear 58 (which also functions asan inter-drive-gear 58) has twenty-seven teeth. Thus, when the inputshaft speed is +1000 rpm and the inter-shaft speed is −610 rpm in LOWspeed mode, the speed of the wheel-ring 47 is 610*27/73=+226 rpm.Similarly, when the input shaft speed is +1000 rpm and the inter-shaftspeed is −1000 rpm in HIGH speed mode, the speed of the wheel-ring 47 is1000*27/73=+370 rpm.

Persons of skill in the art will appreciate that no reverse gear isprovided in the presently described embodiment that would permit thevehicle 100 to travel in a reverse direction. This function can beprovided separately from the presently described transmission embodimentthrough the variable speed drive 114. Alternatively, when the primemover 130 is an electric motor, an opposite rotational direction of theinput shaft 23 can be readily provided by reversing the direction of themotor, resulting in a reverse direction of movement of the vehicle 100.

The inter-shaft 38 includes an inter-shaft extension 37 that extendsbeyond the inter-shaft-bearing 39R, outwardly of the housing 20. Theinter-shaft extension 37 can be fitted with a parking brake in the formof a mechanical brake or high drive ratio electric brake motor that canbe selectively electrically shorted to provide a high resistance toback-driving. Since the inter-shaft 38 is solid with inter-drive-gear58, which is meshed with wheel-drive-gear 45, preventing rotation of theinter-shaft 38 in this manner effectively prevents rotation of thewheel-ring 47. In the embodiment shown in FIGS. 1-2 and 4-6, no parkingbrake structure is provided; however, in FIG. 3, a parking brake 31 isschematically illustrated by way of dashed lines. Aninter-shaft-sprocket 35 is provided on the inter-shaft 38 and isrotationally free relative thereto to function as an idler for thesteer-chain 75. When the parking brake 31 is applied to prevent rotationof the inter-shaft 38, the steer-chain 75 is free to move, which allowsrotations of the steer-shaft 27 and on-the-spot equal rotationalmovement in opposite directions of the drive-shafts 25L,25R. The parkingbrake 31 thus effectively locks the drive differential 42 againstforward or rearward movement of the vehicle 100, but permits rotation ofthe drive-shafts 25L,25R under influence of the steering motor 165.

The operation of the electric steering motor 165 and its function in thesteering system will now be described. The steering motor 165(illustrated schematically by dashed lines in FIG. 3 but omitted fromFIGS. 1-2 and 4-6 for clarity) is connected to the steer-shaft 27rotationally supported within the housing 20 (omitted from FIGS. 1-6 forclarity, as previously noted) by steer-shaft-bearings 28L,28R. Thesteering motor 165 can readily rotate the steer-shaft 27 in either aforward or reverse direction. The steer-shaft 27 solidly carries asteer-gear 76 and a steer-sprocket 77. The steer-gear 76 is in mesh witha drive-steer-gear 78 and the steer-sprocket 77 is connected to adrive-steer-sprocket 79 by an endless steer-chain 75, causing them torotate in the same direction as the steer-shaft 27. The meshedsteer-gear 76 and drive-steer-gear 78 rotate in opposite directions.Thus, regardless of the direction of rotation of the steer-shaft 27, thedrive-steer-gear 78 and drive-steer-sprocket 79 rotate in oppositedirections to one another. The ratio of the number of teeth on thesteer-gear 76 and drive-steer-gear 78 is substantially equivalent to theratio of the diameters of the steer-sprocket 77 and drive-steer-sprocket79 so that rotation of the steer-shaft 27 causes equal and oppositerotation of the drive-steer-gear 78 and drive-steer-sprocket 79. Therelationship between the afore-mentioned components is illustratedschematically in FIGS. 4-6, which show the components of the speedvarying structure of the transmission in a planar relationship with oneanother. In order to make the transmission more compact, a planarrelationship is not used in practice.

Continuing with the numerical example, in the currently describedembodiment, the steer-gear 76 has forty-five teeth and thedrive-steer-gear 78 has eighty-one teeth.

Persons of skill in the art will understand that the steer-sprocket 77,drive-steer-sprocket 79 and endless steer-chain 75 could be replaced bygears that are both enmeshed with an idler to ensure that they bothcontinue to rotate in the same direction. In this case, the ratio of thenumber of teeth on the gears and idler are selected so as to achieveequal rotation to the combination of the steer-gear 76 anddrive-steer-gear 78, as previously described.

Continuing with the numerical example of the current embodiment, thesteer-gear 76 has forty-five teeth and the drive-steer-gear 78 haseighty-one teeth. Therefore, the drive-steer-gear 78 rotates at 45/81 ofthe speed of the steer-shaft 27 in the opposite direction. Thus, instraight-ahead motion with the steer-shaft 27 rotating at +1000 rpm, thedrive-steer-gear 78 rotates at 1000*45/81=−556 rpm. Thedrive-steer-sprocket 79 rotates at +556 rpm. It is desirable that thesteer ratio, defined as the number of rotations of the steer-shaft 27 tothe drive-steer-gear 78 and drive-steer-sprocket 79, is in the range offrom 1.5:1 to 2.5:1, preferably substantially 2:1. In the numericalexample given above, the steer ratio is 1000/556=1.8.

The operation of the drive-differential 42 will now be described withreference to FIG. 7. The drive differential 42 comprises left and rightdrive-differential portions sharing a common drive-differential-housing,being the said wheel-ring 47. The wheel-ring 47 carries thewheel-drive-gear 45. The wheel-drive-gear 45 is, as mentioned, in meshwith the inter-drive-gear 58 that is solid with the inter-shaft 38. Thewheel-drive-gear 45 remains in this same fixed ratio with respect to theinter-shaft 38 during forward motion, whether HIGH or LOW is selected,and whether or not the vehicle is being steered.

Drive-sun-gears 83L,83R of the drive-differentials are solid with, androtate with, the drive-steer-gear 78 and drive-steer-sprocket 79,respectively. Left and right drive-spiders 85L,85R are solid with theleft and right drive-shafts 25L,25R, which extend through the housing 20and are rotationally supported by left and right drive-shaft-bearings98L,98R for ultimate connection to the drive structures 105.Drive-sun-gears 83L,83R are supported by bearings and allowed to freelyrotate relative to the left and right drive-shafts 25L,25R.Drive-ring-gears 80L,80R, are provided circumferentially about an innersurface of the wheel-ring 47 for both the left and right drivedifferential portions. A plurality of drive-planet-gears 84L,84R (fourfor each of the right and left drive-differential portions, althoughonly two are shown for each drive-differential portion in FIG. 7) arerotationally mounted on corresponding drive-stub-shafts 86L,86Rprojecting outwardly from the drive-spiders 85L,85R. Thedrive-planet-gears 84L,84R are enmeshed with both the drive-ring-gears80L,80R and the drive-sun-gears 83L,83R.

A centre-pin 90 is screwed into the left drive-shaft 25L, and becomesrigidly unitary therewith. The centre-pin 90 runs in acentre-pin-bearing 92 with respect to the right drive-shaft 25R. Thetaper-roller drive-ring-bearings 96 thus are mounted on the centre-pin90, rather than directly on the drive-shafts themselves, permitting thewheel-ring 47 to rotate about the centre-pin 90 under the influence ofwheel-drive-gear 45.

Rotation of the wheel-ring 47 about the centre-pin 90 causes rotation ofthe drive-planet-gears 84L,84R about the drive-stub-shafts 86L,86Rthrough action of the drive-sun-gears 83L,83R. The drive-stub-shafts86L,86R in turn urge the drive-spiders 85L,85R to rotate and thus causerotation of the left and right drive-shafts 25L,25R, albeit at adifferent rate than that of the wheel-ring 47 that is dependent upon theintervening gear ratio. The speed of the left and right drive-shafts25L,25R when no steering input is provided can be considered a nominalrotation speed or base rotation speed of the drive-differential 42.Although the drive-sun-gears 83L,83R are also urged to rotate by virtueof being enmeshed with the drive-planet-gears 84L,84R, since thedrive-steer-gear 78 and drive-steer-sprocket 79 are constrained torotate at equal speeds and in opposite directions to one another throughthe steer-shaft 27, their tendency to move is cancelled out. Thus, thedrive-sun-gears 83L,83R remain stationary in the absence of steeringinput from the steering motor 165.

However, in the presence of input power from the steering motor 165causing the steer-shaft 27 to rotate in the same direction as thewheel-ring 47, the drive-steer-gear 78 is caused to rotate in theopposite direction, thereby increasing the rotational speed of the leftdrive-planet-gears 84L, causing them to orbit more quickly, therebyincreasing the rotational speed of the left drive-spider 85L and alsothe left drive-shaft 25L. This causes the vehicle 100 to turn to theright. Simultaneously, the opposite effect is taking place on the rightside due to the influence of the drive-steer-sprocket 79, which isrotating in the same direction as the wheel-ring 47. This causes theright drive-planet-gears 84R to orbit more slowly, thereby decreasingthe rotational speed of the right drive-spider 85R and the rightdrive-shaft 25R. In fact, if the input from the steer-shaft 27 causesthe drive-steer-sprocket 79 to rotate fast enough, the orbit of theright drive-planet-gears could even reverse direction and cause thedrive-shaft 25R to spin in the opposite direction to the drive-shaft25L. This further enhances the turning of the vehicle 100 to the right.Of course, rotating the steer-shaft 27 in the opposite direction to thewheel-ring 47 would cause the vehicle 100 to turn to the left.

The equation or formula that relates the speeds of the variouscomponents of an epicyclic gearset can be expressed as follows. Anepicyclic gearset includes a sun-gear, and planet-gears that mesh withthe sun-gear. The gearset also includes a ring-gear havinginwards-facing teeth, which mesh with the planet-gears. The planet-gearsare mounted on respective legs of a rotating spider. The sun-gear, thespider, and the ring-gear, are mounted for relative rotation about acommon axis. The formula is:R _(spider) =R _(ring) *T _(ring)/(T _(ring) +T _(sun))−R _(sun) *T_(sun)/(T _(ring) +T _(sun)),

where the prefix R refers to a rotary speed (e.g. in rpm) of the statedcomponent and the prefix T refers to the number of teeth on the statedgear.

In the numerical example of the current embodiment, in respect of theleft and right drive-differential portions, T_(ring) is the number ofteeth in the drive-ring-gears 80L,80R, being seventy-nine teeth, andT_(sun) is the number of teeth in the drive-sun-gears 83L,83R, beingthirty-five teeth. Thus, for this transmission, the above formulabecomes:R _(drive-spider) =R _(wheel-ring)*79/(79+35)−R _(drive-sun)*35/(79+35),i.e.R _(drive-spider) =R _(wheel-ring)*0.693−R _(drive-sun)*0.307.

Again, the speeds of the various shafts are computed on the basis thatthe input shaft 23 is rotating at +1000 rpm. To recap, with no steering,the drive-steer-gear 78 and drive-steer-sprocket 79 rotate at 0 rpm, dueto the requirement for opposite rotation through the steer-shaft 27.Drive-sun-gears 83L,83R of the drive-differentials are solid with, androtate with, the drive-steer-gear 78 and drive-steer-sprocket 79,respectively. Thus, unsteered, the two drive-sun-gears 83L,83R bothrotate at 0 rpm. These figures can be entered in the above formula asthe R_(sun) term.

The R_(ring) term in the formula is the speed of the wheel-ring 47,which is common to the two drive-differential portions, the speed being+226 rpm in LOW mode and +370 rpm in HIGH mode.

Left and right drive-spiders 85L,85R are solid with the left and rightdrive-shafts 25L,25R. Now, the speeds of the left and rightdrive-spiders 85L,85R can be determined, upon entering the R_(ring) andR_(sun) figures in the above formula. Thus, for the numerical example ofthe current embodiment in straight ahead motion, the formula(R_(spider)=R_(ring)*0.693−R_(sun)*0.307) in respect of bothdrive-differentials, computes as:

1) in unsteered HIGH mode,R_(drive-spider-unsteered-high)=370*0.693−0*0.307=256−0=+256 rpm; and,

2) in unsteered LOW mode,R_(drive-spider-unsteered-low)=226*0.693−0*0.307=156−0=+156 rpm.

Again, the drive-spiders 85L,85R are solid with the drive-shafts25L,25R, so these computed speeds of the drive-spiders are also thespeeds of the drive-shafts.

The corresponding computations in respect of steering the vehicle willnow be addressed. Assuming an input of +1000 rpm to the steer-shaft 27and using the previously determined steer ratio of 1.8:1, thedrive-steer-gear 78 rotates at 1000/1.8=556 rpm, but in the oppositedirection to the steer-shaft 27, making it −556 rpm. Similarly, thedrive-steer-sprocket 79 rotates at +556 rpm.

Thus, in the left portion of the drive-differential 42, the leftdrive-sun-gear 83L rotates at −556 rpm (HIGH or LOW); i.e.R_(leftsun-steered)=−556. In the right portion of the drive-differential42, the right drive-sun-gear 83R rotates at +556 rpm (HIGH or LOW); i.e.R_(rightsun-steered)=+556.

So now, with the input shaft 23 rotating at +1000 rpm, the above formula(R_(drive-spider)=R_(wheel-ring)*0.693−R_(wheel-sun)*0.307) computes as:

1) in steered HIGH mode, right sideR_(rightdrive-spider-steered-high)=370*0.693−(+556*0.307)=256−(+171)=+85rpm and left side R_(leftdrive-spider-steered-high)=370*0.693(−556*0.307)=256−(−171)=+427 rpm; and,

2) in steered LOW mode, right sideR_(rightdrive-spider-steered-low)=226*0.693−(+556*0.307)=156−(+171)=−15rpm and left side R_(leftdrive-spider-steered-low)=226*0.693(−556*0.307)=156−(−171)=+327 rpm.

Since the drive-spiders 85L,85R are solid with the drive-shafts 25L,25R,these computed speeds of the drive-spiders are also the speeds of thedrive-shafts. Hence, for a steer-shaft input of +1000 rpm, the speed ofthe right drive-shaft 25R is less than that of the left drive-shaft 25Land the vehicle 100 turns to the right. Rotation of the steer-shaft 27in the opposite direction would cause the vehicle 100 to turn to theleft.

The steering radius is, in LOW mode, considerably tighter than it is inHIGH mode. As can be seen from the above numerical example, at asteer-shaft input speed of +1000 rpm, in LOW mode the drive-shaft 25Ractually turns in the opposite direction to the drive-shaft 25L, causingthe vehicle to execute a sharp right turn. In HIGH mode, although thedifference between the speeds of the right drive-shaft 25R and the leftdrive-shaft 25L is the same (171 rpm), since the nominal or base speedof the drive-shafts 25L,25R is higher (256 rpm vs. 156 rpm), both shaftscontinue to turn in the same direction. Thus, the nominal or base speedof the drive-shafts 25L,25R is varied by a steer ratio (in the numericalexample, 1.8:1) of the steer-shaft 27 upwardly and downwardly, dependingon the direction of the input to the steer-shaft 27. The speed of thedrive-shafts 25L,25R is increased or decreased from nominal by a numberof rotations that is directly related to the rotational speed of thesteer-shaft 27 by the steer ratio (in this case 1.8).

One of the benefits of the present transmission design is that thewasted energy usually associated with skid-steering can be reduced. Inaddition, a more gradual turn can be achieved as compared withconventional skid steering, wherein drive structure on one side isabruptly changed to affect the turn. By ramping up the speed of thesteering motor 165 through the steering control structure 160, aprogressively increasing rate of turn can be achieved, which is morenatural for vehicle operators and provides greater flexibility in termsof vehicle operation. Also, rotating the slower of the two drive-shaftsslowly backwards during a tight forward turn is effective to prevent, orat least to inhibit, the drive structure (wheels or track) on the slowside from tending to slip and slide. It is when the slow-side drivestructure is actually stationary that the treads or cleats tend tobecome clogged with clinging slippery ground material, and then start toslip.

The steer-motor 165 provides additional power to the steering system,over and above that provided by the prime move 130. In conventionalskid-steered vehicles, brakes are utilized to slow drive structure onone side or the other of the vehicle. This reduces the overall poweravailable to the vehicle through friction and results in the vehicleslowing through the turn, sometimes causing the operator tocounter-intuitively increase the power of the prime mover 130 (e.g. byopening a throttle) during the turn. In contrast, using the steer-motor165 does not remove overall power from the system through frictionallosses and instead increases the total available power during steering.This manner of arranging the steering system of the vehicle contributesto making it possible for the present system to be of simple, robustconstruction and to be suitable for energy-efficient use in acomparatively low-powered vehicle. It has been found that a vehicleequipped with the steering/transmission system as described herein canbe expected to use greater than thirty percent (e.g. 35 or 40%) lessenergy on average than a corresponding vehicle that is otherwisesimilarly equipped, but employs simple skid-steering.

The wheel-ring 47 floats on the rotating left and right drive-shafts25L,25R, without requiring a large external bearing for support. Thewheel-ring 47 is supported by drive-ring-bearings 96 upon and betweenthe two rotating wheels-shafts 25L,25R, which are themselves supportedon respective drive-shaft-bearings 98L,98R carried by the housing 20.The elements of the drive-shaft-bearings 98 are, as can be seen,well-spaced and very robustly supported in the housing 20, so much sothat floating the wheel-ring 47 from and between the drive-shafts 25 canbe utilized.

Of course, the left drive-shaft 25L rotates at a different speed fromthe right drive-shaft 25R during steering, and the components must bestructured to accommodate that fact, while providing the requiredsupport. The centre-pin 90 is screwed into the left drive-shaft, andbecomes rigidly unitary therewith. The centre-pin 90 runs in thecentre-pin-bearing 92 with respect to the right drive-shaft 25R. Thetaper-roller drive-ring-bearings 96 thus are mounted on the centre-pin90, rather than directly on the drive-shafts themselves.

By resorting to what might be called the floating-shafts-within-shaftssupport system, as described, for the wheel-ring 47, the transmissionand steer system as a whole is much simplified as a structural unit. Ifthe wheel-ring 47 had to be supported on bearings which were themselvescarried directly by the housing 20, the structure as a whole would haveto be considerably widened, and made heavier, and would becomesignificantly more complex. In a transmission, small increases in size,weight and increased complexity can be regarded as large multipliers ofcost and fuel consumption.

Herein, two shafts or other rotating entities are described as being “ina fixed-ratio relationship” if the gear ratio between them solelydictates the ratio of their relative rotational speed. Thus, two shaftsare not in a fixed-ratio relationship: (a) where the two shafts arerelatively-rotatable components of an epicyclic train; or, (b) where thetwo shafts are relatively-rotatable components of a differential.

The expression is generally applicable when the constancy of the ratioremains unaffected by whether or not the vehicle is being steered. Wherethe speed ratio between two shafts changes during steering maneuvers ofthe vehicle, those two shafts are not in a fixed-ratio relationship.However, the fact that the gear ratio of two shafts is capable of beingchanged (e.g. manually) from one fixed ratio to another, as an isolatedevent, does not prevent the gearing relationship between the two shaftsfrom being described as a fixed-ratio relationship.

For example, the steer-shaft 27 is not in a fixed-ratio relationshipwith the inter-shaft 38 or the input shaft 23; however, the inter-shaft38 and the input shaft 23 are in a fixed ratio relationship when eitherHIGH or LOW mode is selected (but not in neutral). The leftdrive-sun-gear 83L is in a fixed-ratio relationship with the leftdrive-shaft 25L and the right drive-sun-gear 83R is in a fixed-ratiorelationship with the right drive-shaft 25R. The wheel-ring 47 is in afixed-ratio relationship with the inter-shaft 38 and with the inputshaft 23 when either HIGH or LOW mode is selected (but not in neutral).The two drive-shafts (i.e. the left drive-shaft 25L and the rightdrive-shaft 25R) are not in a fixed ratio relationship when steering,but otherwise can be considered to be in a fixed ratio relationship.

The expression “fixed ratio” means that the ratio does not change duringsteering. However, it should be noted that, in the case of some of thepairs of rotating entities in the gearbox of the drawings, the fixedratio does change upon shifting between HIGH mode and LOW mode. Thus, inthe embodiment shown, the fixed-ratio in HIGH mode between the inputshaft 23 and the wheel-ring 47 is different from the fixed-ratio betweenthose two components in LOW mode, but their ratio, whether that ratio isderived from HIGH mode or LOW mode, does remain constant so long as themode remains unchanged.

A “bearing”, as that term is used herein, is a device in which ananti-friction means (e.g. balls/rollers/needles, or low-frictionmaterial) is in direct contact with two races, being an inner race andan outer race. The races are, or are attached rigidly to, respectiveelements or components of the apparatus, being elements that are capableof rotary movement relative to each other. (One of the elements might bestationary.) Often, the races are attached to their respective elementsby being an interference fit therein; but whatever the manner ofattachment, the race is said to be integrated into the element insofaras the race functions as if it were formed directly into the element.

Some of the components and features in the drawings have been givennumerals with letter suffixes, which indicate left or right versions ofthe components. The numeral without the suffix is used herein toindicate the component generically.

Terms of orientation (e.g. “left”, “right”, etc.) when used herein areintended to be construed as follows. The terms being applied to anapparatus, that apparatus is distinguished by the terms of orientationonly if there is not one single orientation into which the apparatus, oran image or mirror image thereof, can be placed, in which the terms canbe applied consistently.

The accompanying drawings are diagrammatic. In respect of some of thecomponents that are shown monolithically, for ease of explanation ofcomplex operation, of course the designer would see to it that thecomponents are divided up into two or more elements, for ease ofmanufacturing or assembly purposes.

Herein, the term “unitary” is used to refer to two components which,during operation, perform as if they had been manufacturedmonolithically, although the components may be separable ordismantlable, e.g. for manufacturing or assembly or servicing reasons.

The novel features will become apparent to those of skill in the artupon examination of the description. Inventive combinations of any onedisclosed feature with another disclosed feature are intended to beclaimed by the inventors. It should be understood, however, that thescope of the claims should not be limited by the embodiments, but shouldbe given the broadest interpretation consistent with the wording of theclaims and the specification as a whole.

The invention claimed is:
 1. A skid-steered vehicle comprising: a primemover; a continuously variable transmission (CVT) coupled to primemover; and a transmission, the transmission having: an input shaftrotatably coupled to the CVT and configured to be rotatably driven bythe CVT, the input shaft having an input-high-gear and an input-low-gearrotatably selectably coupled thereto; an inter-shaft having rotatablycoupled thereto an intermediate-high-gear and an intermediate-low-gear,the inter-shaft further extending through an inter-shaft-sprocket,wherein the intermediate-high-gear is intermeshed with theinput-high-gear and the intermediate-low-gear is intermeshed with theinput-low-gear; a wheel-ring, the wheel-ring having a wheel-drive-gearthat is solid with the wheel-ring, the wheel-drive-gear beingintermeshed with the intermediate-high-gear; a steer shaft in parallelwith the input shaft, the steer shaft configured to be rotatably drivenby a steering motor, wherein the steer shaft carries a steer-gear and asteer-sprocket, the steer-gear and the steer-sprocket being rotatablycoupled to the steer shaft, wherein the steer-gear is intermeshed with adrive-steer-gear that rotates in a direction opposite to that of thesteer shaft, and wherein the steer-sprocket is connected to adrive-steer-sprocket via a steer-chain, the drive-steer sprocket rotatesin the same direction as the steer-sprocket, and theinter-shaft-sprocket meshes with the steer-chain; and, left and rightdrive shafts epicyclically connected to both the wheel-ring and thesteer shaft, the left and right drive shafts urged to rotate in the samedirection by rotation of the wheel-ring and urged to rotate in oppositedirections by rotation of the steer shaft, wherein a drive-sun-gear issolid with and rotates with the drive-steer-gear.
 2. The transmission ofclaim 1, wherein rotation of the wheel-ring urges the steer shaft toremain stationary in the absence of being rotatably driven by thesteering motor.
 3. The transmission of claim 1, wherein the left andright drive shafts are rotatable at a nominal speed and wherein rotationof the steer shaft varies the speed of the left or right drive shaftupwardly or downwardly relative to the nominal speed by a ratio of thespeed of the steer shaft, wherein the ratio is in the range of 0.5 to3.5.
 4. The transmission of claim 3, wherein the speeds of the left andright drive shafts are simultaneously varied in opposite directionsrelative to the nominal speed.
 5. The transmission of claim 1, whereinthe wheel-ring is rotatable by the input shaft at a first speed, thesteer shaft is rotatable by the steering motor at the first speed andwherein the left and right drive shafts are rotatable at two differentspeeds, both less than the first speed.
 6. The transmission of claim 1,wherein the steering motor is an electric motor able to rotate the steershaft in a forward or reverse direction.
 7. The transmission of claim 1,wherein the steer ratio, defined as the number of rotations of the steershaft to the drive-steer-gear, is in the range of 1.5:1 to 2.5:1.
 8. Thetransmission of claim 1, wherein the drive-sun-gear is able to freelyrotate relative to the left or right drive shaft.
 9. The transmission ofclaim 1, wherein a drive-ring-gear is provided circumferentially aboutan inner surface of the wheel-ring and wherein a plurality ofdrive-planet-gears are rotationally mounted on correspondingdrive-stub-shafts projecting from a drive-spider solid with the left orright drive shaft, the drive-planet-gears enmeshed with both thedrive-ring-gear and the drive-sun-gear.
 10. The transmission of claim 9,wherein rotation of the wheel-ring causes rotation of thedrive-planet-gears about the drive-stub-shaft through action of thedrive-sun-gear and wherein the drive-stub-shafts in turn cause thedrive-spider to rotate.
 11. The transmission of claim 10, wherein thesteeling shaft is rotated in the same direction as the wheel-ring by thesteering motor, thereby increasing the rotational speed of thedrive-planet-gears and causing them to orbit more quickly, therebyincreasing the rotational speed of the drive-spider.
 12. The steeringsystem of claim 1, wherein a steering command interface is remote fromthe vehicle and connected wirelessly thereto.
 13. The vehicle of claim1, wherein the vehicle is autonomous.
 14. The transmission of claim 1,wherein the drive-sun-gear rotates independently of the prime mover.